On FnDef type annotation suggestion, use fn-pointer output

Partly addresses #71209.

src/test/ui/suggestions/fn-needing-specified-return-type-param.rs

@@ -0,0 +1,5 @@
+fn f<A>() -> A { unimplemented!() }
+fn foo() {
+    let _ = f; //~ ERROR type annotations needed for `fn() -> A`
+}
+fn main() {}

src/test/ui/suggestions/fn-needing-specified-return-type-param.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed for `fn() -> A`
+  --> $DIR/fn-needing-specified-return-type-param.rs:3:13
+   |
+LL |     let _ = f;
+   |         -   ^ cannot infer type for type parameter `A` declared on the function `f`
+   |         |
+   |         consider giving this pattern the explicit type `fn() -> A`, where the type parameter `A` is specified
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.